In remote control systems, a serial digital data stream is often used to transmit data between a transmitter and a receiver. In most cases, amplitude shift keying is used. The digital information is used to turn a transmitter on (logic 1) or off (logic 0). In some cases, the digital information is transmitted using the frequency shift keying method (different frequency for logic 1 and 0). In both cases, the digitally modulated signal is detected by the receiver, and demodulated, i.e. a slightly different voltage is generated for a logic 1 and a logic 0. The demodulated signal can be very small and noisy. A signal shaper circuit is used to recover digital information from that weak, serial data stream.
Typically, the demodulator circuitry produces a fixed DC level (operating point) when no modulated signal is present. When a modulated signal (usually RF) is received, the demodulator reacts by producing corresponding variations about its operating point. The amplitude of these variations vary depending on the strength of the transmitted signal, the distance between transmitter and receiver, gain and selectivity of the receiver. In typical applications, the variations at the output of the demodulator are of the order of a few hundred mV down to about one mV.
In order to perform further processing (decoding) of the serial data stream in the digital domain, signal shaping is required. The shaping operation consists of transforming the small signal variations into clean, full-level digital signals. The operation is complicated by the fact that typically, a certain amount of noise is added to the small, demodulated signal. Another complication comes from the fact that the operating point of the demodulator may not be well defined, or may even drift as a result of temperature variations or aging.
Traditionally, simple signal shaping is performed using a self-biasing comparator, as shown on prior art FIG. 14. (This is sometimes also referred to such as an adaptive filter, although strictly speaking, this terminology is incorrect.) The unshaped signal is applied to the non-inverting input of the comparator, while a low-pass filtered (i.e. averaged) version of the same signal is applied to the inverting (or reference) input. This method has been used by the Chamberlain Group, Inc. in their garage door opener receivers and by TRW, Inc. in their remote keyless entry receivers.
The prior art of FIG. 14 uses the low-pass filtered version of the signal as an approximation of the operating point of the comparator. It is clear that in order to obtain good result from averaging, the time constant of the low-pass filter must be longer than one bit time (the duration of the shortest pulse in the serial data stream).
There are several disadvantages to this method:
1. The common-mode range of the comparator is normally limited. This places restrictions on the DC biasing point of the channel signal. As a result, circuits that use this method are less flexible in their potential uses.
2. If the received signal level (i.e. variation about the DC operating point) is smaller than the comparator offset, variations at the input will not cause a change in comparator output. As a result, the sensitivity (i.e. smallest signal that can be reliably recognized) of this method is limited by the comparator offset.
To increase the sensitivity, it is possible to place an amplifier stage before the comparator as illustrated in prior art FIG. 15. However, the input to the amplifier stage must be referenced with respect to the operating point of the demodulator (OP), or the operating point at the output will drift, possibly beyond the allowable input range of the comparator. This means that adding an amplifier stage at the input does not fundamentally solve the operating point problem.
It is accordingly an object of the invention to provide a signal shaping circuit having increased sensitivity which addresses the operating point problem.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having the benefit of the specification and drawings herein.